linux-kernel - Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth

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Date:	Tue, 2 Feb 2016 21:53:10 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@....com>
Subject: Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth

On Wed, 27 Jan 2016 15:44:22 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Jan 26, 2016 at 01:52:19PM +0100, luca abeni wrote:
> 
> > > The trouble is with interfaces. Once we expose them we're stuck
> > > with them. And from that POV I think an explicit SCHED_OTHER
> > > server (or a minimum budget for a slack time scheme) makes more
> > > sense.
> 
> > I am trying to work on this.
> > Which kind of interface is better for this? Would adding something
> > like /proc/sys/kernel/sched_other_period_us
> > /proc/sys/kernel/sched_other_runtime_us
> > be ok?
> > 
> > If this is ok, I'll add these two procfs files, and store
> > (sched_other_runtime / sched_other_period) << 20 in the runqueue
> > field which represents the unreclaimable utilization (implementing
> > hierarchical SCHED_DEADLINE/CFS scheduling right now is too complex
> > for this patchset... But if the exported interface is ok, it can be
> > implemented later).
> > 
> > Is this approach acceptable? Or am I misunderstanding your comment?
> 
> No, I think that's fine.
So, I implemented this idea (/proc/sys/kernel/sched_other_period_us
and /proc/sys/kernel/sched_other_runtime_us to set the unreclaimable
utilization), and some initial testing seems to show that it works fine.

However, after double-thinking about it I am wondering if using a
runqueue field to store the unreclaimable utilization (unusable_bw in my
original patch) makes sense or not... This value is the same for all
the runqueue, and changing sched_other_runtime/sched_other_period
changes the unreclaimable utilization on all the runqueues... So maybe
it is better to use a global variable instead of a runqueue field?

Any ideas / suggestions? Before sending a v2 of the RFC, I'd like to
be sure that I am doing the right thing.


			Thanks,
				Luca